1. Field of the Invention
The invention pertains to the production of printed circuit boards. More particularly, the invention concerns the production of high density built-up multilayer circuit boards by constructing microvias with photoimageable dielectric materials.
2. Description of the Prior Art
As the need for faster, smaller, less expensive integrated circuit products continues grow, the ability to wire-bond reaches the limits of the available technology and chips must be mounted using a flip-chip approach and solder bumps. This leads to a direct chip attachment package. The requirement to fan-out the high number of I/O's from the underside of the chip places increasing demands on utilization of the printed circuit board area. Plated-through-holes use too much space and block routing channels. This drives the need for a high density package with a significant number of interconnections on the outer surface of the board as well as for increasing use of blind microvias.
Resin coated copper (RCC) has been used in the past to economically fabricate high density built-up multilayer circuit boards. Currently microvias in such circuit boards fabricated with RCC are produced by two methods, including plasma etching and laser drilling. As such, only printed circuit fabricators with access to plasma etching or laser drilling equipment can provide these advanced, blind-via boards. The high cost of the plasma and laser equipment hinders widespread adoption of RCC technology. Furthermore, the technical disadvantages associated with the plasma etching and laser drilling techniques, such as undercutting due to isotropic etching of plasma, and low throughput due to sequential drilling by laser, also limit large scale commercialization of RCC based high density multilayer circuit boards.
Alternatively, photovia processes, which use photoimageable dielectric materials to fabricate builtup multilayer printed circuit boards have been developed. In theses processes, photodielectrics are coated on a patterned core and photoimaged to define via holes. The via holes along with the surface of the dielectric layer are then plated with copper. U.S. Pat. No. 5,354,593 sequentially laminates and photoimages two photodielectrics onto a conductive core to define via holes and then copper plates the via holes. U.S. Pat. No. 5,451,721 produces a multilayer printed circuit board by applying a photosensitive resin layer onto a core having a metal line on its surface. After imaging to form via holes, the resin layer is deposited with a copper layer by electroless plating techniques. U.S. Pat. No. 5,334,487 produces a pattered layer on a substrate by applying and exposing different photosensitive compositions on opposite sides of a copper foil. One side is developed and the copper etched, followed by developing the other side and metallization of through holes.
The foregoing photovia technologies allow for fabrication of high density interconnection printed circuit boards with conventional equipment but they suffer from similar drawbacks such as difficult copper plating processes and poor resin-to-copper adhesion. These problems usually lead to poor reliability of the circuit boards. These problems are solved by the present invention whereby a photosensitive dielectric composition on a conductive foil is laminated to conductive lines on a substrate. After imaging the foil, and imaging and curing the photosensitive dielectric composition, vias are formed to the conductive lines. Thereafter the conductive lines are connected through the vias to the conductive foil, and then the conductive foil is patterned.